It is known in the art that the electron emitters of a field emission display can become contaminated and/or dulled due to bombardment by ions. Such ions typically are liberated from the cathodoluminescent phosphors when the phosphors are activated by the field-emitted electrons.
One prior art scheme for addressing this problem of ion bombardment is taught in U.S. Pat. No. 5,543,691, entitled "Field Emission Display with Focus Grid and Method of Operating Same", by Palevsky et al. Palevsky et al teach providing a focus grid disposed between the anode and the cathode. Palevsky et al teach that the focus grid can have an aperture that is offset from the electron emitters, so that the field-emitted electrons will be imaged to a point on the phosphor anode that is not in line with the focus grid aperture. Palevsky et al teach that ions, which are generated at the anode, are intercepted by the focus grid and do not reach the cathode.
However, a disadvantage of this prior art scheme is that the focus grid has a large number of electrons impinging on it, and these electrons tend to generate low voltage secondary electrons. A percentage of these secondary electrons drift into the focus grid aperture and are then accelerated by the anode. Palevsky et al teach reducing the effect of secondary electrons by coating the surface of the focus grid with a material that has a reduced secondary emission coefficient. However, this solution adds an additional fabrication step in the fabrication of the focus grid.
Palevsky et al further teach reducing the effect of secondary electrons by placing a second focus grid between the focus grid and the anode. This solution adds additional fabrication and alignment steps in the fabrication of the field emission display. Also, Palevsky et al teach that in the operation of the display, the second focus grid is biased negatively with respect to the first focus grid, so that the low energy secondary electrons from the first focus grid are repelled by the second focus grid while the higher energy electrons from the cathode will have enough energy to pass through to the anode. Thus, the electrons from the cathode are made less energetic. By reducing the velocity of the electrons, the brightness of the display is compromised.
Furthermore, the focus grid described by Palevsky et al is not connected to the cathode. Rather, support is only provided along the edges of the focus grid. Thus, the focus grid may be prone to bowing or sagging.
Accordingly, there exists a need for an improved field emission display, which overcomes at least these shortcomings of the prior art.